Generally, in a plasma processing apparatus, plasma of a processing gas is generated within a decompression processing vessel. Further, a thin film is formed on a processing target object within the processing vessel by a gas phase reaction or a surface reaction of radicals or ions included in the generated plasma, or micro-processing such as etching of a material or a thin film on a surface of the processing target object is performed.
For example, a capacitively coupled plasma processing apparatus includes an upper electrode and a lower electrode arranged in parallel to each other within a processing vessel. A processing target object (e.g., a semiconductor wafer, a glass substrate, etc) is mounted on the lower electrode, and a high frequency power having a frequency (typically, 13.56 MHz or higher) suitable for plasma generation is applied to the upper electrode or the lower electrode. Electrons are accelerated in a high frequency electric field generated between the two facing electrodes by applying the high frequency power, and plasma is generated as a result of ionization by collision between the electrons and a processing gas.
Recently, as a design rule is getting more miniaturized in a manufacturing process of a semiconductor device or the like, higher level of dimensional accuracy is required in, especially, the plasma etching. Further, it is required to increase etching selectivity against a mask or an underlying film and to improve etching uniformity in the entire surface of a substrate. For this reason, a pressure and ion energy in a processing region within a chamber tends to be reduced, so that a high frequency power having a high frequency equal to or higher than 40 MHz is used.
However, as the pressure and the ion energy are reduced, an influence of a charging damage, which has been negligible conventionally, can be no more neglected. That is, in a conventional plasma processing apparatus having the high ion energy, no serious problem may occur even when a plasma potential is non-uniform in the entire surface of the substrate. However, if the ion energy is lowered at a lower pressure, the non-uniformity of the plasma potential in the entire surface of the substrate may easily cause the charging damage on a gate oxide film.
In this regard, to solve the above-mentioned problem, a method of pulse-modulating a high frequency power for plasma generation with an on/off (or H level/L level) pulse having a controllable duty ratio (hereinafter, referred to as “first power modulation method”) has been considered effective. According to this first power modulation method, a plasma generation state in which plasma of a processing gas is being generated and a plasma non-generation state in which the plasma is not being generated are alternately repeated at a preset cycle during a plasma etching process. Accordingly, as compared to a typical plasma process in which plasma is continuously generated from the beginning of the process to the end thereof, a time period during which plasma is continuously generated may be shortened. As a result, the amount of electric charges introduced into a processing target object from the plasma at one time or the amount of electric charges accumulated on the surface of the processing target object may be reduced, so that the charging damage is suppressed from being generated. Therefore, a stable plasma process can be performed and reliability of the plasma process can be improved.
Further, conventionally, in the plasma processing apparatus, a RF bias method is widely employed. In this RF bias method, a high frequency power having a relatively low frequency (typically, 13.56 MHz or lower) is applied to the lower electrode on which the processing target object is mounted, and ions in plasma are accelerated and attracted to the processing target object by a negative bias voltage or a sheath voltage generated on the lower electrode. In this way, by accelerating the ions in the plasma and bringing them into collision with the surface of the processing target object, a surface reaction, an anisotropic etching or a film modification may be facilitated.
However, when performing the etching process to form via holes or contact holes by using the plasma etching apparatus, a so-called micro-loading effect may occur. That is, an etching rate may differ depending on the hole size, so that it is difficult to control an etching depth. Especially, the etching rate tends to be higher at a large area such as a guide ring (GR), whereas the etching rate tends to be lower at a small via into which CF-based radicals are difficult to be introduced.
In this regard, to solve the above-stated problem, a method of pulse-modulating a high frequency power used for ion attraction with an on/off (or H level/L level) pulse having a controllable duty ratio (hereinafter, referred to as “second power modulation method”) has been considered effective. According to the second power modulation method, a pulse-on period during which an on-state (or H-level state) of a relatively high power suitable for etching a preset film on the processing target object is maintained and a pulse-off period during which an off-state (or L-level state) of a relatively low power (a high frequency power for ion attraction) suitable for depositing polymer on a preset film on the processing target object is maintained are alternately repeated at a certain cycle. Accordingly, at an area having a larger hole size, a proper polymer layer may be deposited on the preset film at a higher deposition rate, so that the etching may be suppressed. Thus, an undesirable micro-loading effect may be reduced, and it may be possible to perform an etching process with a high selectivity and a high etching rate.
Patent Document 1: Japanese Patent Laid-open Publication No. 2000-071292
Patent Document 2: Japanese Patent Laid-open Publication No. 2012-009544
Patent Document 3: Japanese Patent Laid-open Publication No. 2013-033856
When using the first power modulation method or the second power modulation method in a plasma process, a high frequency power to be pulse-modulated varies in a step-shape between the on-state (or H-level state) and the off-state (or L-level state) of a modulation pulse, so that plasma of a load greatly pulsates periodically. Accordingly, in a matching device that transfers a high frequency power as a continuous wave without undergoing power modulation to the load (plasma) within a chamber, a matching operation may not be performed stably, and it may be difficult to achieve expected effect of the corresponding power modulation methods.
By way of example, in the second power modulation method, on the assumption that a high frequency power for plasma generation as a continuous wave without the power modulation is stably supplied to plasma at a regular input power, it is expected that an etching process is performed during the pulse-on period and deposition of a reaction product or exhaust and removal of a reaction by-product is performed during the off-period. Here, the input power of the high frequency power is a net power obtained by subtracting a power of a reflection wave or a power of a high frequency power supply, from a power of a progressive wave and is also called a “load power.”
If, however, the plasma of load varies between the pulse-on period and the pulse-off period as stated above, the power of the reflection wave may also vary on a high frequency transmission line for transmitting the high frequency power for plasma generation as the continuous wave, so that the input power may be changed. In this case, if the input power of the high frequency power for plasma generation is greatly reduced during the pulse-on period, the etching process may not be performed as expected. If the input power of the high frequency power for plasma generation is greatly reduced during the pulse-off period, the plasma may be unstable.
Further, conventionally, the matching device that transmits a high frequency power as a continuous wave without the power modulation to the load samples voltage detection signals and electric current detection signals obtained on the high frequency transmission line of the high frequency power with the same sampling frequency during the pulse-on period and the pulse-off period, and calculates a load impedance measurement value corresponding to an arithmetic average of the total samples within a single cycle. Then, the matching device controls a variable reactance element within a matching circuit, e.g., varies a capacitance of a variable capacitor such that the load impedance measurement value (arithmetic average value or moving average value thereof) can be made equal to or approximate to a matching point corresponding to an output impedance of a high frequency power supply. Accordingly, the degree of matching may differ between the pulse-on period and the pulse-off period depending on the lengths of the periods or the lengths of the sampling time (magnitude of the number of the samples). The degree of matching in a longer period may be relatively closer to a fully matched state than that in a short period. From another point of view, in the pulse-on period and the pulse-off period, an offset amount of a load impedance during a relatively longer period from the matching point is lower than an offset amount of a load impedance during a relatively shorter period.
If the duty ratio of the modulation pulse is varied in the power modulation method, a ratio between the lengths of the pulse-on period and the pulse-off period may be changed, and the load impedance measurement value (matching target point) which is matched with the matching point may also be changed by being affected by the change of the duty ratio. Accordingly, a balance between the offset amount of the load impedance during the pulse-on period and the offset amount of the load impedance during the pulse-off period from the matching point may be changed in the matching operation. As a result, a ratio between an input power during the pulse-on period and an input power during the pulse-off period may be changed.
As stated above, conventionally, it has been difficult to control the ratio between the input powers during the pulse-on period and the pulse-off period for the high frequency power as a continuous wave without the power modulation regardless of the duty ratio of the pulse modulation.